Scientists have discovered a group of microbes, named after Marvel characters Thor, Odin and Heimdall, that may explain how complex cellular life on the Earth emerged from simpler microbial ancestors billions of years ago. Life on our planet can be divided into three major groups. Two of these groups are represented by tiny microbes, the Bacteria and the Archaea.

The third group of organisms comprises all visible life, such as humans, animals and fungi – known as eukaryotes. While the cells of bacteria and archaea are small and simple, eukaryotes are made up of large and complex cells. The origin of these complex cell types has long been a mystery to the scientific community, but now researchers led by Uppsala University in Sweden have identified a group of microorganisms that provides a unique insight into the evolutionary transition from simple to complex cells.

It is known that eukaryotes at some point shared a common ancestor with archaea. It was also clear that symbiosis – a process involving an intimate collaboration between two cell types – played an important role in this process. Most scientists share the view that a symbiosis in which an archaeal host cell took up a bacterium ultimately gave rise to eukaryotes.

Yet, whether this symbiosis was the cause or rather the consequence of the evolution of complex cells remained an open question. Researchers discovered of a new group of Archaea, the Asgard archaea, which reveal important details on how eukaryotic cells evolved their complexity.

“By using new methods to obtain genome data from microbes that cannot be grown in the laboratory, we identified a new archaeal group that is related to the host cell from which eukaryotic cells evolved,” said Thijs Ettema at Uppsala University.

In an earlier study, researchers described genomic data for ‘Loki’, an archaeon living in the ocean floor that represented the closest living micro-organism of complex cellular life. In the current study, which corroborates these previous findings, several new Loki-related archaea are described.

“These organisms are our closest microbial relatives, and we know next to nothing about them. Current methods allow us to take a first genetic sneak peek,” said Ettema. “Our findings are based on analysis of genetic material that was directly obtained from the environment. We have actually never seen these cells,” said Jimmy Saw, researcher at Uppsala University.

“We named these new archaea Thor, Odin and Heimdall after the Norse gods, and together with Loki, they form the Asgard archaea,” said Eva Fernandez-Caceres from Uppsala University. “These new groups are found in various environments all over the world, and not only in the deep sea, as Loki. So far they are most abundant in sediments,” said Fernandez-Caceres.

The study provides strong evidence that eukaryotes evolved from a lineage that was related to these Asgard archaea. The study was published in the journal Nature.